Effects of 5 minute cerebral ischemia due to bilateral occlusion of the common carotid arteries were studied in 3-week old and adult gerbils. The evaluation of cerebral blood flow with l4C iodoantipyrine radioautography revealed severe, uniform (below l0 ml/l00g/min) ischemia in most of the both hemispheres, similar in intensity in both young and adult gerbils. However, biochemical assays revealed a considerable difference concerning the rate of depletion of the main energy metabolites indicating a slower energy metabolism in the young gerbils. Morphological studies carried out after 2 weeks revealed no evident ischemic injury in the young animals, whereas the brain of adult gerbils showed characteristic severe destruction of the CA1 sector of the hippocampus. These studies indicate that the thresholds for ischemic injury are age-dependent and that the young animals show a lesser sensitivity than adult ones to ischemic brain damage.